1. Field of the Invention
This invention relates, generally, to microstereolithography. More particularly, it relates to a non-degenerate two-photon approach to projection microstereolithography.
2. Description of the Prior Art
Microstereolithography enables the manufacturing of small and complex three-dimensional components from plastic materials.
One-photon polymerization is a process that causes a photo-initiator monomer concentration to induce a photochemical reaction, which in turn causes the concentration to cross-link and solidify. The process is the basis for most commercially available stereolithography systems.
Two-photon polymerization is a technique for the fabrication of three dimensional micron and sub-micron structures. A beam of ultra fast infrared laser is focused into a container holding a photo-sensitive material to initiate the polymerization process by non-linear absorption within the focal volume. By focusing the laser in three dimensions and moving the laser through the resin, a three dimensional structure can be fabricated. Two-photon microstereolithography enables three dimensional processing as well as high complexity microfabrication.
Researchers have demonstrated experimental two-photon micro/nano stereolithography but have not incorporated projection technology into the two-photon fabrication process and have not combined non-degenerate two-photon photopolymerization based on intersecting femtosecond pulsed projected images with picosecond pulsed laser light sheet at the focal plane. Existing two-photon stereolithography techniques enable unlimited complexity in the part geometries that can be fabricated by polymerizing a single focal volume voxel inside the bulk volume of photopolymer via the two-photon absorption process. However, these systems are limited in the volume that can be fabricated in a timely manner due to the point-by-point fabrication approach. These systems also require ultra-precision control of translation or mirror steering systems to generate parts of adequate resolution at the micro scale. The trend of everincreasing two-photon absorbing cross-sections of photoinitiators explicitly tailored for two-photon processes in recent years suggests that the speed of the scanning mirror systems will also present some limitations in two-photon stereolithography now and in the future.
One-photon based microstereolithography techniques fabricate in a surface layer-by-layer approach that ultimately limits the process to rapid prototyping and some small production runs of micropolymer structures. The surface layer-by-layer approach also limits the geometries of objects that can be fabricated due to surface tension or release layer issues, and requires an extensive network of support structure to be digitally inserted into three-dimensional models via support structure insertion algorithms. All of these factors limit the fabrication process and slows the overall throughput of micropolymer structures.
There also exists a gap between prototyping of complex micro geometries using microstereolithography and mass production of complex geometries. The ideal microstereolithography device would allow any complexity in geometry, need no support structure, and enable rapid prototyping, mass-production, and mass customization from a single machine.
Two-photon absorption can occur in two forms: degenerate and non-degenerate. The process is known as degenerate if the photons absorbed are of the same wavelength. The process is known as non-degenerate when the photons absorbed are of two-different wavelengths.
Nearly all of the research conducted on two-photon polymerization has been limited to degenerate schemes using a single focused laser beam. Non-degenerate two-photon polymerization, using two lasers of two different wavelengths, increases set-up costs, requires optical hardware having a more complex configuration and dual laser pulse synchronization. However, a non-degenerate configuration offers distinct advantages that have an impact on the overall throughput and versatility of the fabrication system. Non-degenerate systems offer more control over the geometry of the reaction volume due to the fact that the reaction volume is confined only to the overlapping beams of the appropriate wavelengths.
The rate of degenerate two-photon absorption, in a dual intersecting beam degenerate two-photon configuration, increases where the two beams intersect but photo-absorption also occurs in the light path prior to the desired reaction volume if the beams enter a sample already tightly collimated, or at a low numerical aperture. This configuration causes some two-photon absorption (TPA) in the beam delivery paths with an increase in absorption occurring at the intersection of the two beams, thus limiting the overall irradiance that is deliverable to the desired fabrication volume. This situation also limits the achievable speed of photopolymerization and feature size resolution. For two-photon polymerization photon absorption in the beam's delivery path is an undesired effect and is solved by implementing a focusing scheme with a high numerical aperture. The increase in the probability for absorption to occur as the beam approaches the focal point reduces the possible degenerate configurations to designs that have a high numerical aperture objective lens.
Thus there is a need for a two-photon projection microstereolithography method that incorporates a non-degenerate two-photon approach to projection micro stereolithography but which is not subject to the limitations of the known methods.
However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in this art how the identified needs could be met.